2026 ICD-10-CM Diagnosis Code R99Ill-defined and unknown cause of mortality

ICD-10-CM Codes›R00–R99›R99›R99

ICD-10-CM R99
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

R99 is a billable ICD-10-CM diagnosis code for ill-defined and unknown cause of mortality. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026). As a symptom code, it should not be used as a principal diagnosis once a related definitive diagnosis has been established. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Other general signs and symptoms.

Code Identity

ICD-10-CM Code
R99
Billable Status
Yes — Valid for Submission
Code Describes
Ill-defined and unknown cause of mortality
Chapter
R99
Ill-defined and unknown cause of mortality

Code Classification

ChapterR00–R99Symptoms, signs and abnormal clinical and laboratory findings, not elsewhere classified
SectionR99Ill-defined and unknown cause of mortality
CategoryR99Ill-defined and unknown cause of mortality
This CodeR99Ill-defined and unknown cause of mortality

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Accidental death
  • Accidental death in home
  • Accidental death in industrial place
  • Accidental death in public place
  • Accidentally killed
  • Adverse incident outcome
  • Adverse incident resulting in death
  • Anesthetic death
  • Antepartum maternal death
  • Autopsy pending
  • Autopsy performed
  • Child death
  • Clavien-Dindo classification finding
  • Clavien-Dindo classification grade V
  • Dead
  • Dead - death without witness
  • Dead - expected
  • Dead - sudden death
  • Dead - suspicious death
  • Dead - unexpected
  • Dead on arrival at hospital
  • Death
  • Death associated with influenza
  • Death associated with respiratory syncytial virus infection
  • Death by asphyxiation
  • Death by fire
  • Death by hanging
  • Death by immolation
  • Death due to chemotherapy toxicity
  • Death due to radiotherapy toxicity
  • Death due to trampling
  • Death during anesthetic induction
  • Death from direct obstetric cause occurring more than 42 days but less than one year after delivery
  • Death from indirect obstetric cause occurring more than 42 days but less than one year after delivery
  • Death from overwork
  • Death from sequela of direct obstetric cause
  • Death from sequela of indirect maternal cause
  • Death in home
  • Death in hospice
  • Death in hospital
  • Death in hospital-based emergency department
  • Death in hospital-based emergency department or outpatient department
  • Death in hospital-based outpatient department
  • Death in less than 24 hours from onset of symptoms
  • Death in long term care facility
  • Death in nursing home
  • Death in nursing home or long term care facility
  • Death of unknown cause
  • Death unattended by physician
  • Died at home
  • Died during operation
  • Died in ambulance
  • Died in care home
  • Died in community hospital
  • Died in general practice premises
  • Died in hospice
  • Died in hospital
  • Died in learning disability unit
  • Died in mental health unit
  • Died in nursing home
  • Died in street
  • Died in usual place of residence
  • Died without sign of disease
  • Direct maternal death
  • Early neonatal death
  • ECOG performance status - grade 5
  • ECOG performance status finding
  • Fetal disorder due to maternal death
  • Found dead
  • Found dead at accident site
  • Found dead in bed
  • GRACILE syndrome
  • Iatrogenic death
  • Indirect maternal death
  • Infant death
  • Instantaneous death
  • Intraoperative death
  • Intraoperative or immediately postoperative death in an ASA Class I patient
  • Intrapartum maternal death
  • Killed
  • Late maternal death
  • Late neonatal death
  • Maternal death
  • Maternal death - perinatal
  • Natural death
  • Natural death reportable to medicolegal authority
  • Natural death with probable cause suspected
  • Natural death with proved cause
  • Natural death with proved cause by autopsy
  • Natural death with proved cause without autopsy
  • Natural death with unknown cause
  • Neonatal death
  • Non-traffic vehicular accidental death
  • Patient death associated with a fall while being cared for in a healthcare facility
  • Patient death or serious disability associated with a burn incurred from any source while being cared for in a healthcare facility
  • Patient death or serious disability associated with an electric shock while being cared for in a healthcare facility
  • Patient death or serious disability associated with patient elopement for more than four hours
  • Patient death or serious disability associated with the use of contaminated drugs, devices, or biologics provided by the healthcare facility
  • Patient death or serious disability associated with the use of restraints or bedrails while being cared for in a healthcare facility
  • Patient death or serious disability associated with the use or function of a device in patient care, in which the device is used or functions other than as intended
  • Patient deceased during stay
  • Patient status determination, deceased
  • Patient status determination, deceased and body cremated
  • Patient status determination, deceased and body donated
  • Patient status determination, deceased and buried
  • Patient status determination, deceased and entombed
  • Patient status determination, deceased and removed from state
  • Perinatal death
  • Perioperative death
  • Postmortem plain X-ray abnormal
  • Postmortem rigidity
  • Postneonatal death
  • Postoperative death
  • Postpartum maternal death
  • Preoperative anesthetic death
  • Serious reportable event associated with environment
  • Serious reportable event associated with patient protection
  • Serious reportable event associated with product or device
  • Sudden death
  • Sudden death of unknown cause during the puerperium
  • Sudden death, cause unknown
  • Sudden infant death
  • Sudden infant death syndrome
  • Sudden infant death with dysgenesis of testes syndrome
  • Sudden unexpected death in epilepsy
  • Suspicious death
  • Traffic vehicular accidental death
  • Unattended death
  • Unattended death of unknown cause
  • Undetermined manner of death
  • Undetermined manner of death, natural causes suspected
  • Undetermined manner of death, suicide suspected
  • Unexpected death
  • Unexpected sudden death of adult
  • Unexplained sudden death

Tabular List NotesGuidance

Coding notes and annotation back-references applicable to this code.

Inclusion Terms

  • Death (unexplained) NOS
  • Unspecified cause of mortality

Index to Diseases and InjuriesGuidance

Alphabetical index entries that point to this code.

External Cause of Injuries IndexGuidance

References for this code in the External Cause of Injuries Index.

    • Cot death
    • Crib death
    • Death(cause unknown) (of) (unexplained) (unspecified cause)
    • Sudden infant death syndrome(SIDS)
    • Syndrome
      • crib death

Clinical ClassificationClinical

AHRQ’s CCSR groups this code into broader clinical categories.

CCSR SYM016
Other general signs and symptoms
Default principal diagnosis: inpatient Yes · outpatient Yes

Clinical InformationClinical

  • Perinatal Death

    the death of a fetus of gestational age 28 weeks or more, or the death of a live-born infant less than 28 days of age.

  • Sudden Infant Death

    the abrupt and unexplained death of an apparently healthy infant under one year of age, remaining unexplained after a thorough case investigation, including performance of a complete autopsy, examination of the death scene, and review of the clinical history. (pediatr pathol 1991 sep-oct;11(5):677-84)

  • Infant Death

    the death of a live-born infant within its first year of life.

  • Maternal Death

    the death of the female parent.

  • Sudden Unexpected Death in Epilepsy

    sudden death in a patient with epilepsy associated with seizures and seizure-related symptoms (e.g., apnea; hypoxemia) without other identifiable accidental causes (e.g., drowning; wounds and injuries).

  • Infant

    a child between 1 and 23 months of age.

  • Direct Maternal Death

    death resulting from conditions or complications which are unique to pregnancy and occur during the antepartum, intrapartum, or postpartum period.

  • Indirect Maternal Death

    death resulting from previously existing disease or a disease developing during pregnancy which was not associated with gestation, but which was aggravated by the unique physiologic changes of pregnancy. examples of indirect deaths include those associated with epilepsy, diabetes, cardiac disease and hormone-dependent malignancies.

Convert R99 to ICD-9-CMHistory

The closest ICD-9-CM equivalents under the General Equivalence Mappings.

ICD-9-CM
798.0 Sudden infant death synd
Approximate The match is approximate rather than exact.
ICD-9-CM
799.9 Unkn cause morb/mort NEC
Approximate The match is approximate rather than exact.

Code HistoryHistory

FY 2016AddedAdded to the ICD-10-CM code setEffective October 1, 2015, the first year of ICD-10-CM.
FY 2017–2025No changes
FY 2026CurrentCurrent code set, no changesEffective October 1, 2025 through September 30, 2026.

Questions About R99Overview

Is R99 (Ill-defined and unknown cause of mortality) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report ill-defined and unknown cause of mortality on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

Can R99 be a principal diagnosis?

Use it with care. This is a symptom code, so once a definitive diagnosis explaining the ill-defined and unknown cause of mortality is established, that condition takes the principal position instead.

What is the ICD-9 equivalent of R99?

Under the General Equivalence Mappings, ill-defined and unknown cause of mortality converts to ICD-9-CM 798.0 (sudden infant death synd) and 799.9 (unkn cause morb/mort NEC). The mapping is approximate, so confirm the match fits the documentation.

Footnotes

[1] Not chronic - A diagnosis code that does not fit the criteria for chronic condition (duration, ongoing medical treatment, and limitations) is considered not chronic. Some codes designated as not chronic are acute conditions. Other diagnosis codes that indicate a possible chronic condition, but for which the duration of the illness is not specified in the code description (i.e., we do not know the condition has lasted 12 months or longer) also are considered not chronic.